But exercising subjective discretion over the quality of sites is a thorny topic for the world’s dominant search engine, which already faces antitrust scrutiny over charges it uses its dominance to undermine competitors.

As such, it seemed worth exploring how Google’s rivals fared under the changes.

SISTRIX analyzed how the visibility of many sites shifted before and after the search tweak, by evaluating how often particular domains showed up against a set of one million keywords. It produced a list (updated and expanded since the link above) of the 100 sites that declined the most and the 68 that climbed the highest.

Our informal review found that Google rivals fell into both columns. Whether it tilted one way or the other on the whole depends largely on how competition is defined, but it didn’t immediately appear to lean overwhelmingly in either direction.

The losers included Associated Content, a site that isn’t a direct competitor to Google, but which chief rival Yahoo bought last year for $90 million. It lost 93 percent of its visibility in search rankings, according to SISTRIX.

Another was Merchant Circle, which competes against Google Places, by also offering listings of local businesses. It was 21st on the list, with an 85 percent drop.

Mahalo, a so called human-powered search engine, dived 84 percent.

(Some other sites could be seen as competitors depending on how broadly one defines it. A number of the demoted properties offer shopping comparisons or information about specific topics, both of which Google also does, though through very different approaches.)

Merchant Circle of Mountain View wasn’t immediately available for comment.

Yahoo, which has rebranded Associated Content as the Yahoo Contributor Network, responded with a prepared statement that stressed it expects traffic to come from its own sites.

“The Yahoo Contributor Network is committed to surfacing its highest quality content and contributors to the largest possible audience, and the focus of this strategy is on distribution across Yahoo’s leading media properties and products,” the statement read. “While off-network distribution is constantly evolving, we believe the Yahoo Contributor Network remains the best platform for contributors to reach audiences at scale, including the hundreds of millions of people who visit Yahoo.”

Though Google didn’t use the term, the major change to its algorithm was widely seen as targeting what have become known as “content farms.” These sites specialize in throwing up quick stories, videos or how to instructions on popular topics, often relying heavily on material from other publications.

As we wrote last week:

Though the quality is often low, the material frequently ranks highly in search results thanks to the use of headlines and popular keywords that catch the attention of search algorithms. Consumers and technology critics have increasingly complained that the material clutters up search results, and undermines the value of Google.

Overall, the consensus seems to be that the tweak has caught the very sort of sites that Google was targeting. (Though there have been some difficult to understand “civilian casualties” as well, as noted here and here.)

“Comparing these results with the announcement from Google, they seem to have reached their goal: a whole lot of low-quality domains lost significant visibility,” concluded Johannes Beus of SISTRIX on the company’s blog.

Without commenting on any specific site, a quick review of a handful of the demoted listings by The Chronicle generally found the low quality characteristics that Google had said it was looking for, including “sites with shallow or poorly written content, content that’s copied from other websites, or information that people frankly don’t find that useful.”

There was considerable curiosity over how Demand Media would fare, one of the most prominent companies tagged with the content farm label. It adamantly rejects the term.

The company generally fell into the decliner column, with sites like Livestrong.com, Answerbag.com and Trails.com losing visibility. But one of its most prominent properties, eHow.com, climbed up 15 percent.

Google Fellow Amit Singhal and Principal Engineer Matt Cutts said of the changes in a blog post Thursday: “We can’t make a major improvement without affecting rankings for many sites. It has to be that some sites will go up and some will go down.”